UV radiation (254 nm) induces pyrimidine dimers in DNA of mammalian cells, which are to a varying extent photoreactivable. Photoreactivating light itself induces in DNA binding sites of unknown nature for the photoreactivating enzyme, presumably through the action of hydrogen peroxide produced by the illumination. Work in the coming year will include the publication of part of the experimental work summarized in this report, which requires repetition and expansion of several kinds of experiments. Furthermore, the studies hitherto performed with marsupial cornea cells will be extended to cells of placental mammals and humans. These studies will concern in particular (a) the cell survival after irradiation with 254 nm or greater than 360 nm radiation; (b) the formation of binding sites for the photoreactivating enzyme in DNA by wavelengths greater than 360 nm and by hydrogen peroxide; and (c) the effects of sunlight (unfiltered and filtered) on mammalian cells and DNA regarding lethal damage and photorepair.